1. Field of the Invention
This invention relates to engine control systems and, in particular, to a method and system for controlling various devices in an internal combustion engine based on variations in engine operating conditions.
2. Discussion of Related Art
In recent years advances such as variable cam timing, variable valve lift, and charge motion control have introduced additional degrees of freedom and increased complexity to engine control. In response, an engine control system has been developed known as the “lockstep” system in which devices in the engine are simultaneously controlled without explicitly compensating for the interdependence between the devices.
In the lockstep system, a series of points are chosen spanning the engine speed/engine load domain. At each point, various devices are adjusted until the combination of settings provides a desired result. For example, one result might be “stability limited” (SL) operating conditions in which optimal fuel economy and optimal emission levels are attained for a predetermined ambient temperature. Another result might be “optimal power” (OP) operating conditions in which an optimal output torque (i.e., wide open throttle) is attained for a predetermined altitude. Another result might be “default” (D) or “limp home” (LH) operating conditions resulting from, e.g., locked camshaft actuators which occasionally result from certain conditions such as cold temperatures or low oil pressure in the engine. Control values for each device in the engine are scheduled according to these determinations.
The lockstep system has one drawback. The control value schedules are generally determined for the various engine devices in a fixed operating environment that does not account for variations in environmental operating conditions such as ambient air temperature, engine coolant temperature, engine oil temperature and humidity. Instead, these environmental operating conditions are assumed to be constant during control value scheduling. Optimum values for controlling various engine devices, however, may vary responsive to changes in environmental operating conditions. For example, variable cam position may be limited by engine oil temperature.
The inventors herein have recognized a need for a method and system for controlling a device in an internal combustion engine that will minimize and/or eliminate one or more of the above-identified deficiencies.